1. Field of the Invention
The present invention relates to sound field correcting apparatus and method for falsely producing a good acoustic space by adding reflected sounds and reverberation sounds to an original signal sound.
2. Description of the Related Background Art
When the reverberation characteristics in the acoustic space of a concert hall or the like are shown by an impulse response, they are as shown in, for instance, FIG. 1. Although the impulse response changes in dependence on the environment of the acoustic space, it can be separated to the initial reflected sound portion and the following reverberation sound portion on the basis of the level and density. The initial reflected sounds are called elements to determine a spatial image such as a feeling of extent in the room of a concert hall or the like and have high levels. As for the reverberation sounds subsequent to the initial reflected sounds, the density increases in proportion to the square of the time and the sounds are repetitively reflected by the wall surfaces in the room, so that there is a tendency such that the high frequency components decrease.
There is a sound field correcting apparatus, on the other hand, to falsely produce an acoustic space like a concert hall at home or in the room of a vehicle by adding the reflected sounds and reverberation sounds to the original signal sound. In the conventional sound field correcting apparatus, a delay circuit group 1 which is constructed by serially connecting a plurality of delay circuits as shown in FIG. 2 is provided. An input digital audio signal is supplied to the first delay circuit of the delay circuit group 1. The audio signal delayed by the delay circuit group 1 is supplied to a coefficient multiplier group 2. The coefficient multiplier group 2 multiplies a coefficient to the delayed audio signal by each coefficient multiplier and supplies signals indicative of the results of the multiplications to an adder 3. A signal indicative of the result of the addition of the adder 3 is further supplied as a reflected sound signal to an adder 5 through a coefficient multiplier 4. The input digital audio signal as an original signal is supplied to the adder 5 through a coefficient multiplier 6, so that a reflected sound signal is added to the original signal and the resultant addition signal is generated from the adder 5 as an output signal.
FIG. 3 shows impulse response characteristics in case of using such a conventional sound field correcting apparatus. When comparing this impulse response characteristics with the impulse response in the actual acoustic space shown in FIG. 1, the characteristics in both of the initial reflected sound portions are almost equal. In the reverberation sound portion, however, the density in case of using the sound field correcting apparatus is lean. This is because such a lean density state occurs since there are limitations of the numbers of stages of the delay circuits and the coefficient multipliers. Since the density is lean, there are problems such that the lingering tone of the sound doesn't smoothly decrease in terms of the sense of hearing and what is called a feeling of extent which is sensed in, for example, a concert hall is not obtained and a feeling of physical disorder is sensed.
To raise the density of the reverberation sounds, on the other hand, for example, the use of a recursive type filter represented by a comb filter is considered. In this case, however, since an extremely large number of arithmetic operations are repeated, the signal is deteriorated and an adverse influence is, consequently, exerted on the sound quality. In case of using the recursive type filter, there is a drawback such that a limit cycle in which the output signal does not become 0 although the input signal is set to 0 occurs.